It is well known that cis-1,4-polybutadiene can be prepared by polymerizing 1,3-butadiene monomer with nickel based catalyst systems. Such nickel based catalyst systems contain (a) an organonickel compound, (b) an organoaluminum compound, and (c) a fluorine containing compound. Such nickel based catalyst systems and their use in the synthesis of cis-1,4-polybutadiene is described in detail in U.S. Pat. No. 3,856,764, 3,910,869, and 3,962,375.
The cis-1,4-polybutadiene prepared utilizing such nickel based catalyst systems typically has a high molecular weight. Due to this high molecular weight, the cis-1,4-polybutadiene is generally oil extended. However, this precludes the cis-1,4-polybutadiene from being utilized in many applications. For instance, such oil extended rubbers cannot be utilized in tire sidewalls which contain white sidewall compounds. In any case, there is a large demand for cis-1,4-polybutadiene having a reduced molecular weight which can be processed without being oil extended.
Various compounds have been found to act as molecular weight reducing agents when used in conjunction with the nickel based catalyst system. For instance, U.S. Pat. No. 4,383,097 discloses that alpha-olefins, such as ethylene and propylene, act as molecular weight reducing agents when utilized in conjunction with such three component nickel catalyst systems. Canadian Patent 1,236,648 indicates that 1-butene, isobutylene, cis-2-butene, trans-2-butene, and allene act as molecular weight regulators when used in conjunction with such nickel based catalyst systems. U.S. Pat. No. 4,383,097 reveals that certain nonconjugated diolefins, such as 1,4-pentadiene, 1,6-heptadiene, and 1,5-hexadiene, act as molecular weight reducing agents when utilized in conjunction with such catalyst systems. U.S. Pat. No. 5,100,982 indicates that cis-1,4-polybutadiene having reduced molecular weight and a broad molecular weight distribution can be synthesized with certain nickel based catalyst systems in the presence of halogenated phenols, such as para-chlorophenol.
The processability of cis-1,4-polybutadiene rubbers can be improved by simply lowering their molecular weight. However, this approach also typically leads to increased cold flow. Accordingly, the use of conventional molecular weight reducing agents, such as .alpha.-olefins, to improve rubber processability leads to compromised cold flow characteristics.